In the current 3rd Generation Partnership Project (3GPP) long term evolution (LTE)-Advanced systems, carrier aggregation (CA) is used to extend communication bandwidths of 1.4, 3, 5, 10, 15, and up to 20 megaHertz (MHz) in Release 8/9 and up to 100 MHz in Release 10. Such large bandwidth communication is achieved by the simultaneous aggregation of more than one component carrier, hence the term carrier aggregation, in which each carrier within the aggregated set of carriers is referred to as a component carrier. Under Release 10, up to five component carriers may be aggregated together to achieve the maximum bandwidth of 100 MHz.
Two types of carriers are discussed in Release 8/9/10: backwards compatible carrier in Release 8/9 and non-backwards compatible carrier in Release 8/9. (See 3GPP TS 36.300 Version 10.4.0, E-UTRA Overall Description (Release 10), June 2011.) A backwards compatible carrier comprises a carrier accessible to user equipments (UEs) of all LTE releases (e.g., earlier releases such as Release 8/9 as well as the current release). A non-backwards compatible carrier comprises a carrier that is not accessible to UEs of LTE Release 8/9, but is accessible to UEs of Release 10 that defines such a carrier. Each of the backward compatible carrier and the non-backwards compatible carrier can operate as a stand-alone carrier or as part of CA as a component carrier. A stand-alone carrier comprises a carrier than can be used alone without other carriers to provide communication between UEs and enhanced Node Bs (eNodeBs or eNBs). A stand-alone carrier can provide the channels and signals of all the physical layers used by the LTE-Advanced system (e.g., physical downlink control channel (PDCCH), physical hybrid automatic repeat request (HARQ) indicator channel (PHICH), physical control format indicator channel (PCFICH), downlink synchronization signals, downlink reference signals, etc.).
An extension carrier, by contrast, comprises a carrier that cannot operate alone and instead must be part of CA set (must be a component carrier within a component carrier set of the CA), in which at least one of the other component carriers in the component carrier set is a stand-alone capable carrier. The extension carrier may be used as extended elements of the stand-alone capable carrier that is already configured or connected to the UEs. Extension carriers typically lack physical layer control channels or signals information or have limited broadcast capabilities compared to stand-alone carriers. For example, extension carriers may not include any physical layer control channels, such as PDCCH, PHICH, or PCFICH, or lack various control signals information such as downlink synchronization signals or downlink reference signals. The missing information and/or capabilities of an extension carrier may be provided by the stand-alone capable carrier within the CA.
Current LTE-Advanced systems (Release 8/9/10) do not support extension carriers. Future releases may benefit from using extension carriers enhance the network efficiency for providing service (on an as-needed basis) in environments or operating conditions where additional service coverage is warranted.